A method for the determination of lysine-derived collagen crosslinks

Modification of amino acid residues in carious dentin matrix

The Maillard reaction between sugar and protein has been postulated as the cause for the browning and arrestment of caries lesions. This reaction has been implicated as the cause for decreased degradability of collagen in vivo. The aim of the present study was to verify the occurrence of the reaction in vivo. Carious and sound dentin samples were taken from extracted human teeth and analyzed for the fluorescence characteristic of the Maillard reaction and oxidation and, by HPLC, for Maillard products. In addition, physiological cross-links were analyzed by HPLC. Oxidation- and Maillard reaction-related fluorescence increased in collagenase digests from carious dentin. Advanced Maillard products (carboxymethyllysine and pentosidine) increased, whereas furosine, a marker for the initial reaction, was not observed consistently. This implies no direct addition of sugars to protein, but rather the addi-tion of smaller metabolites and glycoxidation products. In addition, the physiological cross-links hydroxylysinonorleucine and dihydroxylysinonorleucine decreased in carious dentin. Also for hydroxylysylpyridinoline, a decrease was observed, but not consistently. In conclusion, the caries process modifies amino acids in dentin collagen, which can lead to increased resistance against proteolysis and ultimately to caries arrestment.

Growth changes of collagen cross-linking, calcium, and water content in bone

It has been claimed that the increase in the strength of growing bone is due to increased mineral content. The strength of collagen is based on intermolecular covalent cross-links, and it has also been proposed that cross-link changes increase bone strength. Measurements of the content of calcium, collagen, and water, as well as cross-link analyses, were performed on the tibial cortex of growing dogs. Within the age range studied (8-44 weeks), no changes in calcium content expressed as a percentage of dry bone weight were seen. Collagen content expressed as weight of hydroxyproline per dry bone weight showed a minor reduction during growth. However, water content decreased considerably up to an age of about 25 weeks, which implies a concomitant increase in the amount of bone material. Of the two cross-link main groups, reducible and nonreducible, it is only possible chemically to analyze the reducible. During the final part of the period of growth and mechanical maturation of the bones, the number of reducible cross-links decreases. This indicates a concomitant increase in the more stable nonreducible forms. The possible mechanical relevance of the chemical changes found during growth is discussed.

Collagen crosslinks in human dentin: increasing content of hydroxypyridinium residues with age

The hydroxypyridinium crosslinks of collagen are believed to derive from reducible, divalent crosslinks. To study this concept further, both types of crosslink were quantified as a function of age in dentin, a tissue thought to have minimal collagen turnover. Human (5, 15, 28 and 56 years) and bovine (fetal and adult) root dentin was analyzed by a procedure that measures both hydroxypyridinium and reducible crosslinks on the amino acid analyzer. In human dentin, hydroxypyridinium crosslinks increased with age and became the predominant crosslinks as the two reducible residues, dehydrodihydroxylysinonorleucine and dehydrohydroxylysinonorleucine, diminished. Similarly in adult bovine dentin, hydroxypyridinium residues were sixfold more concentrated than in fetal bovine dentin. Borohydride treatment of tissue did not influence the measured content of hydroxypyridinium residues. The analyses also ruled out natural reduction as a stabilizing reaction for the divalent, reducible crosslinks. Though hydroxypyridinium residues became the major aldehyde-mediated crosslinks of adult dentin collagen, significant levels of reducible crosslinks remain throughout the tooth's adult life.

The composition of the insoluble collagenous matrix of bovine predentine

Predentine obtained from bovine teeth by microdissection was extracted with EDTA and tris-NaCl solutions. The insoluble residue consisted mainly of collagen and resembled dentine collagen in overall amino acid composition. The residue differed in containing no detectable phosphoprotein, a much larger amount of collagen hexose and more non-collagenous glycoprotein, the neutral sugar composition of which was determined. Differences were also observed in the contents of reducible collagen cross-links. Half of the total phosphorus found in the predentine could not be accounted for solely by hydroxyapatite. The remainder was partly soluble and dialysable.